The need in industry for vibration isolation is growing. For example, there is less and less tolerance for environmental vibration in ultraviolet steppers used in semiconductor manufacturing. As the manufacturing of semiconductors and other products becomes more and more precise, the need for suppressing environmental vibration becomes greater and greater.
Many currently available vibration isolation applications that are based upon “soft springs” also require relatively high level of damping. Dampers are often used to reduce vibration amplification at the resonance frequency of the spring, and to minimize distortion generated on the isolated mass by the moving stages, motors, etc. Unfortunately, acceptable levels of damping can be very limited in most of the available systems. One limiting factor can be attributed to the stiffness increase of the combined damper-spring system, which can result in shifting up the resonance frequency of the system, and in decreasing of the gain/frequency function, i.e., the “roll off” slope above the resonance frequency. As a result, there tends to be a significant loss in vibration isolation gain beyond the resonance frequency.
In general, the level of damping can be determined by (i) the settling time, which is directly related to the resonance frequency of the system and the level of vibration amplification at that frequency, (ii) the vibration isolation specification, especially at high frequency, and/or (iii) the damper type (e.g., active, passive). Known examples of passive dampers include dashpot dampers and fluid dampers. Passive dampers are typically used to benefit system vibration isolation at the resonance frequency of the spring. However, since these dampers are usually coupled to the vibrating base platform, for frequencies above resonance frequencies, these dampers can reduce vibration isolation gains by approximately 20 dB per decade.
Active dampers, on the other hand, may include, for instance, voice coil dampers or motor elements. Active dampers may be used to produce relatively high compensation forces, and along with sensors on the isolated payload, can compensate for the forces generated by the heavy payload moved with high acceleration. However, active dampers also have very limited active bandwidth gain. In particular, the coupling of payload resonances with sensed outputs can compromise stability margins. This limitation may be due to the servo loop stability that can be limited by the required attachment of vibration sensors to the isolated platform sensing its multiple resonances.
Generally, a supported payload can often involve moving mechanical components, which can generate dynamic forces that act on the payload and cause it to vibrate in response. The payload, in addition, has a mass that generates a static force. In most existing isolation systems both the static and dynamic forces are permitted to act on a vibration compensation mechanism, for instance, an actuator, and require such compensation mechanism to address both the static and dynamic forces when minimizing vibration. Such an approach requires the use of a very powerful actuator or multiple actuators, both of which can be expensive and bulky. Moreover, finding a compromise between the damping level and vibration isolation gain can be a difficult engineering task.
Accordingly, it is desirable to provide a practicable damping system that can provide relatively high damping forces while at the same time improves vibration isolation.